Composite parts such as panels may be stiffened to resist bending moments using a variety of techniques. One such technique involves forming integral structural features, referred to as “beads”, into the panel during layup. Bead-stiffened panels may be fabricated by laying up prepreg plies on a tool that includes raised bead features on a tool surface that determines the shape into which the panel is formed. The tool may also be used to compact and cure the layup.
Beads of sufficiently effective height may be difficult to form because of the difference in effective length that the fibers in the ply must cover; some fibers extend over the flat areas of the tool while other fibers must extend up and over the bead features. Also ply wrinkling may result in the transition areas between different developed fiber lengths.
One solution to the problem mentioned above involves using larger pieces of prepreg than would otherwise be necessary, which are hand-worked down over the bead features while attempting to smooth out wrinkles and avoid excessive applied tension. Another solution involves sandwiching composite plies between sheets of a carrier material such as aluminum sheet metal which are used to mechanically force the plies down over the beads features while maintaining tension in the sandwich. This technique, however, requires the use of specialized equipment and limits the size of the panel to that of the equipment while requiring more consumable materials that are single use. Moreover, because the forming process is hidden from view until it is completed, problems such as ply wrinkling, deformation or breakage may not be detected until the ply has been formed. Still another method involves using specialized “extra-formable” material forms, however these forms are more costly than standard materials and may not be approved or qualified for use in forming certain types of parts.
Accordingly, there is a need for a method of making bead-stiffened composite parts using standard materials and relatively simple tools which may reduce or substantially eliminate wrinkling and/or unacceptable distortion of ply materials during the bead forming process.